1. Field of the Invention
The present invention relates to a test key and particularly to a test key and method for validating the doping concentration of buried layers within a deep trench capacitors.
2. Description of the Prior Art
Trench capacitors are frequently implemented as the essential charge storage device in a DRAM (Dynamic Random Access Memory). The trench capacitor is formed in the substrate and has a capacitance proportional to the depth of the trench. That is to say, by increasing the depth of the trench, which results in the xe2x80x9cplatesxe2x80x9d occupying a larger surface area, the trench capacitor provides a higher capacitance.
FIG. 1 is a diagram showing the layout of a conventional DRAM. A trench capacitor 10 is disposed below the passing wordline. A transistor 14 is coupled to a node 16 of the trench capacitor 10 through a diffusion region 18. A diffusion region 20 is coupled to a plug 22. The plug 22 is coupled to a bitline (not shown). Thus, data is read from or written into the trench capacitor 10 through the node 16 by operation of the transistor 14. The transistor 14 is controlled by voltages on the wordline 12. When a high voltage level is on the wordline 12, a conductive channel is formed below the wordline 12 so that a current flows from or to the node 16 through the diffusion regions 18 and 20, whereby the data is read from or written into the capacitor 10.
FIG. 2 shows a cross section along the line AA in FIG. 1. An STI (Shallow Trench Isolation) 28 is formed in the substrate and trench capacitor to define an active area and isolate the trench capacitor 10 from the subsequently formed wordline 12. After formation of the wordline 12, the diffusion regions 18 and 20, used as a source and drain, on two sides of the wordline 12 are formed by ion implantation with masking of the wordline 12 and STI 28. The channel length L of the transistor 14 corresponds to the size of the wordline 12 and the profiles of the diffusion regions 18 and 20. Further, the profiles of the diffusion regions 18 and 20 are based on the doping concentration of the storage node 16 composed of conducting layers L13 and L12, and an electrode layer L11. Accordingly, the doping concentrations of the layers L11, L12 and L13 have great impact on the channel length L of the transistor 14. For DRAM employing trench capacitors as storage devices, an invalid doping concentration in the storage node results in current leakage adjacent memory cells or even defective cells. Validation of the doping concentration of the storage node is an essential step for DRAM manufacturing.
The object of the present invention is to provide a test key and method for validating the doping concentration of buried layers within the deep trench capacitor.
The present invention provides a test key comprising a trench capacitor deposited in the scribe line region with an electrode layer of a first doping concentration, a first conducting layer with a second doping concentration and a second conducting layer with a third doping concentration, an isolation region deposited in the trench capacitor, penetrating the second conducting layer and extending into the first conducting layer so that the second conducting layer is divided into a first and second portion, a first plug coupled to a first side of the first portion of the second conducting layer, a second plug coupled to a second side of the first portion of the second conducting layer, and a third plug coupled to the second portion of the second conducting layer.
The present invention further provides a validation method comprising the steps of providing a wafer having at least one scribe line region and a memory cell region, forming a test key in the scribe line region and a plurality of memory cells in the memory cell region, wherein the test key comprises a trench capacitor deposited in the scribe line region with an electrode layer of a first doping concentration, a first conducting layer with a second doping concentration and a second conducting layer with a third doping concentration, an isolation region deposited in the trench capacitor, penetrating the second conducting layer and extending into the first conducting layer so that the second conducting layer is divided into a first and second portion, a first plug coupled to a first side of the first portion of the second conducting layer, a second plug coupled to a second side of the first portion of the second conducting layer, and a third plug coupled to the second portion of the second conducting layer, measuring a first resistance between the first and second plug, measuring a second resistance between the second and third plug, and validating the first, second and third doping concentrations by the first and second resistance.